Showing papers in "Macromolecules in 1996"

Unifying Weak- and Strong-Segregation Block Copolymer Theories

Abstract: A mean-field phase diagram for conformationally symmetric diblock melts using the standard Gaussian polymer model is presented. Our calculation, which traverses the weak- to strong-segregation regimes, is free of traditional approximations. Regions of stability are determined for disordered (DIS) melts and for ordered structures including lamellae (L), hexagonally packed cylinders (H), body-centered cubic spheres (QIm3m), close-packed spheres (CPS), and the bicontinuous cubic network with Ia3d symmetry (QIa3d). The CPS phase exists in narrow regions along the order−disorder transition for χN ≥ 17.67. Results suggest that the QIa3d phase is not stable above χN ∼ 60. Along the L/QIa3d phase boundaries, a hexagonally perforated lamellar (HPL) phase is found to be nearly stable. Our results for the bicontinuous Pn3m cubic (QPn3m) phase, known as the OBDD, indicate that it is an unstable structure in diblock melts. Earlier approximation schemes used to examine mean-field behavior are reviewed, and compa...

Controlled Radical Polymerization of Methacrylic Monomers in the Presence of a Bis(ortho-chelated) Arylnickel(II) Complex and Different Activated Alkyl Halides

Abstract: A novel class of homogeneous nickel(II) catalysts, i.e [Ni{o,o‘(CH2NMe2)2C6H3}Br], denoted as Ni(NCN‘)Br, is reported to mediate in the presence of activated alkyl halides, e.g., CCl4 or α-halocarbonyl compounds, a well-controlled radical polymerization of methacrylic monomers [methyl and n-butyl methacrylate), (MMA, n-BuMA)] at rather low temperatures (<100 °C). The number-average molecular weight of the polymer gradually increased with the monomer conversion and was inversely proportional to the initiator concentration of alkyl halides. The molecular weight distribution (MWD) remained very narrow during the whole course of the polymerization (MWD < 1.3). All the experimental data including a successful block copolymerization (n-BuMA-b-MMA) experiment were in agreement with a living polymerization process, and remarkably enough, poly(methyl methacrylate) (PMMA) with molecular weight up to at least 105 g/mol was synthesized in a controlled fashion. Increased thermal stability of the PMMA is a further indi...

Scattering Functions of Semiflexible Polymers with and without Excluded Volume Effects

Abstract: Off-lattice Monte Carlo simulations on semiflexible polymer chains with and without excluded volume interactions have been performed. The model used in the simulations is a discrete representation of the worm-like chain model of Kratky and Porod applied in the pseudocontinuous limit. The ratio between the cross-section radius R of the chain and the statistical segment length b was chosen to be R/b = 0.1 which corresponds to the value found for polymer-like micelles. The ratio R/b is equivalent to a reduced binary cluster integral of B = 0.30, which is in accordance with the value for polystyrene in a good solvent. The scattering functions of the semiflexible chains have been determined with a precision of 1−2% for L/b = 0.3−640, where L is the contour length of the chain. Numerical approximations to these functions have been determined which interpolate between the simulated functions, and these can be used in the analysis of experimental scattering data. The approximations have been used in least-squares...

Hybrid organic-inorganic thermoplastics : Styryl-based polyhedral oligomeric silsesquioxane polymers

Abstract: A series of linear thermoplastic hybrid materials containing an organic polystyrene backbone and large inorganic silsesquioxane groups pendent to the polymer backbone have been synthesized and characterized. Our preliminary results indicate that the pendent inorganic groups drastically modify the thermal properties of the polystyrene and that interchain and/or intrachain oligomeric silsesquioxane interactions result in variations in solubility and thermal properties

Mechanism of the Stereocomplex Formation between Enantiomeric Poly(lactide)s

Abstract: Poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) crystallize into a stereocomplex with a melting point 50 °C higher than the crystals of the enantiomers. The racemic crystal is formed by packing ‚-form 31-helices of opposite absolute configuration alternatingly side by side. Single crystals of the stereocomplex exhibit triangular shape. The drastic difference of the powder patterns evidences the different packing of the ‚-form in the stereocomplex and in crystals of the pure lactides. By force field simulation of the stereocomplex and the PLLA unit cells and of their powder patterns, the reasons for the different packing could be clarified. Between the ‚-helices in the stereocomplex, van der Waals forces cause a specific energetic interaction-driven packing and, consequently, higher melting point. Helices of identical absolute configuration pack different from pairs of enantiomer ‚-helices. Packing favors R-type helication. A well-defined 103-helix has not been found. Good agreement with the experimental powder patterns proves the correctness of the simulations. On the basis of morphology, packing calculations, and atomic force microscopy, we propose a model of stereocomplex crystal growth, which explains the triangular shape of single crystals. Thus, for polymer components beyond chain folding length, the stereocomplex formation by simultaneous folding of the two types of chains is plausible. The triangular type of crystallizing offers favorable position for the polymer loops during the crystal growth. Our study of the PLA complexation mechanism may offer a chance to predict other polymeric stereocomplexes and their properties.

Origins of Complex Self-Assembly in Block Copolymers

Abstract: Amphiphilic molecules are renowned for their ability to partition chemically immiscible components into nanoscale domains. Often these domains exhibit intriguing complex periodic geometries with long-range order. Surprisingly, the diverse systems that selfassemble in this manner, surfactants, lipids, soaps, and block copolymers, exhibit topologically identical geometries, suggesting to researchers that a common set of principles govern amphiphilic phase selection. From this association has emerged the belief that constant mean curvature (CMC) interfaces are generally good models for block copolymer microdomain geometries. By taking advantage of new developments in polymer theory, we accurately examine this hypothesis for the first time, and find it to be wrong. Furthermore, our study reveals new explanations for complex phase selection that are relevant to numerous block copolymer systems. A block copolymer consists of chemically distinct polymer chains (i.e., blocks) joined together to form a single macromolecule. As a consequence of a general tendency for the blocks to separate, tempered by the restriction imposed by the covalent bonds that connect them, these molecules exhibit amphiphilic behavior. Even in the simplest case, AB diblock copolymers, a rich assortment of ordered phases has been documented.1-8 The composition of the AB diblock (i.e., the volume fraction f of block A) controls the geometry of the structure (see Figure 1). For nearly symmetric diblocks (f ∼ /2), a lamellar (L) phase occurs. For moderate asymmetries, a complex bicontinuous state, known as the gyroid (G) phase, has been observed in which the minority blocks form domains consisting of two interweaving threefold-coordinated lattices.1,2 (Prior to the discovery of the G phase, a double-diamond (D) structure formed from two fourfold-coordinated lattices3 was erroneously associated with the bicontinuous state in these materials.4) Another complex structure, the perforated lamellar (PL) phase, occurs when the minority-component layers of the L phase develop a hexagonal arrangement of passages.5 At yet higher asymmetries, the minority component forms hexagonally packed cylinders (C) and then spheres (S) arranged on a bodycentered cubic lattice. Eventually, as f f 0 or 1, a disordered phase results. The complete mean field or rather self-consistent field theory (SCFT) for block copolymers was developed by Helfand and co-workers.9 However, at the time of its development, it had to be supplemented with approximations limiting its effectiveness. Nevertheless, important advances were made by examining this theory in the limits of weak10 and strong11 segregation. (The degree to which the A and B blocks segregate is determined by the product oN, where o is the FloryHuggins A/B interaction parameter and N is the total degree of polymerization.) The combination of these works established that the underlying physics controlling block copolymer phase behavior involves a competition between interfacial tension and the entropic penalty for stretching polymer coils so as to fill space uniformly. The balance determines the equilibrium size of the microdomains and dictates the geometry of the structure. Although these earlier approaches correctly predicted the classical phases (i.e., L, C, and S),10,11 they failed to account for the more recently discovered complex phases (i.e., G and PL).12,13 With new advances,14 it is now possible to implement the full SCFT. The first calculations14,15 to do so evaluated the free energies of the structures described above and established the phase diagram. This demonstrated that complex phase behavior occurs in the intermediatesegregation regime as opposed to the weakand strongsegregation regimes treated by Leibler10 and Semenov,11 respectively. For intermediate segregation (e.g., oN ) 20), the new calculations predict the sequence L f G f C f S f disordered as f progresses from /2 to either 0 or 1. Although PL is absent from this sequence, it is nearly stable at the L/G phase boundary, consistent with where it is observed experimentally.6 This supports very recent experiments indicating that the PL structure is only a long-lived metastable state. The D phase is clearly unstable, in agreement with current experiments.4,6 Given this theoretical accomplishment, we now probe deeper into the theory14 to examine the physical factors responsible for complex phase behavior. As described below, the explanation lies in the detailed shape of the dividing interface between the A and B microdomains. Earlier works, such as that of Semenov,11 illustrate that the phase transitions are driven by a tendency to curve the interface as the diblocks become asymmetric in composition. The curvature allows the molecules to balance the degree of stretching between the A and B blocks. We demonstrate this quantitatively in Figure * Present address: Polymer Science Centre, University of Reading, Whiteknights, Reading RG6 6AF, UK. Figure 1. Area-averaged mean curvature 〈H〉 as a function of the A-block volume fraction f for each of the structures shown schematically calculated using self-consistent meanfield theory.14,15 The stable and metastable states are shown with solid and dashed lines, respectively, and transitions are denoted by dots. As the molecules become asymmetric, structures with more curvature are preferred. 7641 Macromolecules 1996, 29, 7641-7644

Synthesis of branched and hyperbranched polystyrenes

Abstract: The formation of hyperbranched polymers by a one-pot, atom transfer radical polymerization is described using p-(chloromethyl)styrene (CMS) in the presence of Cu(I) and 2,2'-bipyridyl. The synthesis of branched polystyrene by copolymerization of CMS with styrene is also discussed

Mechanism and Kinetics of Epoxy-Amine Cure Studied by Differential Scanning Calorimetry

Abstract: The isoconversional kinetic analysis has been applied to nonisothermal DSC data on the cure of an epoxynovolac resin. The process reveals a dependence of the activation energy (Eα) on conversion (α). The shape of the dependence has been interpreted in the terms of the reaction mechanisms. It has been found that the model dα/dt = (k1 + αmk2)(1 − α)n used for the kinetically controlled cure gives rise to the dependence of Eα on α similar to the experimentally found one. To completely describe the diffusion-controlled cure, the effect of both T and α on the change in diffusivity has been taken into account. The equation for the specific rate constant of diffusion, kD(T,α) = Do exp(−ED/RT + Kα), has been induced. Its use allows us to obtain a model dependence of Eα on α closely matching the experimental one. A technique of predicting isothermal cure from the sole dependence of Eα on α has been considered.

Compatibilizers for Melt Blending: Premade Block Copolymers†

Abstract: Poly(methyl methacrylate) (PMMA) was melt mixed 30:70 into polystyrene (PS) with and without symmetric P(S-b-MMA) diblock copolymers The molecular weight of the components was varied After 5 min of shear mixing, the PMMA was dispersed into roughly spherical, submicron particles Particle size was measured by light scattering and transmission electron microscopy As little as 1% copolymer led to a significant reduction in PMMA particle size, although larger amounts were needed to make the particles stable to annealing (180 °C for 15 min) The principle role of block copolymers in controlling morphology appears to be in preventing coalescence Preventing dynamic coalescence leads to size reduction, while preventing static coalescence results in stability or compatibilization We estimate that less than 5% of the interface needs to be covered to prevent dynamic coalescence while ∼20% is necessary to impart static stability Mobility, critical micelle concentration, and molecular weight of the block copolym

Cascade of Transitions of Polyelectrolytes in Poor Solvents

Abstract: We develop a scaling model for the dilute solution conformation of a uniformly charged polymer in a poor solvent. We find that there is a range of temperatures and charge densities for which the polymer has a necklace-like shape with compact beads joined by narrow strings. The free energy of a polyelectrolyte in this conformation is lower than in a cylindrical globule because the length of the necklace is larger than that of a cylinder and is proportional to the total charge on the chain. With changing charge on the chain or temperature, the polyelectrolyte undergoes a cascade of abrupt transitions between necklaces with different numbers of beads.

Acid-Catalyzed Polycondensation of Furfuryl Alcohol: Mechanisms of Chromophore Formation and Cross-Linking

Abstract: A thorough investigation of the acid-catalyzed polycondensation of furfuryl alcohol was conducted with the specific aim of understanding the mechanisms responsible for the main reaction, but more p...

Soluble Stoichiometric Complexes from Poly(N-ethyl-4-vinylpyridinium) Cations and Poly(ethylene oxide)-block-polymethacrylate Anions

Abstract: Block ionomer complexes formed between the block copolymers containing poly(sodium methacrylate) (PMANa) and poly(ethylene oxide) (PEO) segments and poly(N-ethyl-4-vinylpyridinium bromide) (PEVP) were investigated. The data obtained suggest that (i) these systems form water-soluble stoichiometric complexes; (ii) these complexes are stable in a much broader pH range compared to the polyelectrolyte complexes prepared from homopolymers; (iii) they self-assemble to form the core of a micelle comprised of neutralized polyions, surrounded by the PEO corona; (iv) they are salt sensitive since they fall apart as the salt concentration increases beyond a critical value; and (v) they can participate in the cooperative polyion substitution reactions. Therefore, these complexes represent a new class of hybrid materials which combine properties of polyelectrolyte complexes and block copolymer micelles.

Morphogenic Effect of Added Ions on Crew-Cut Aggregates of Polystyrene-b-poly(acrylic acid) Block Copolymers in Solutions

Abstract: The morphology of crew-cut aggregates of amphiphilic block copolymers in dilute solutions can be controlled by the addition of ions in micromolar (HCl, NaOH, CaCl2, Ca(Ac)2) or millimolar (NaCl) concentrations. The copolymers are highly asymmetric polystyrene-b-poly(acrylic acid) diblocks, PS-b-PAA, in which the lengths of the insoluble PS blocks are much longer than those of the soluble PAA blocks. In addition to spherical, rodlike, and univesicular or lamellar aggregates, large compound vesicles (LCVs), a new morphology, can be obtained from a single block copolymer. The morphogenic effect of different added ions on the crew-cut aggregates can be ascribed to the changed repulsive interactions among the hydrophilic PAA segments, due to neutralization by NaOH, protonation by HCl, ion-binding or bridging by Ca2+, and electrostatic screening by NaCl, respectively. The formation of the LCVs may involve a secondary aggregation of individual vesicles and a subsequent fusion process. Some features of the sponta...

Synthesis, Characterization, and Hydrolytic Degradation of PLA/PEO/PLA Triblock Copolymers with Short Poly(l-lactic acid) Chains

Abstract: PLA/PEO/PLA triblock copolymers bearing short poly(l-lactic acid) blocks, with the number average degree of polymerization of each PLA block PLA = 2, 4, 8, and 12, were synthesized by ring opening polymerization of l-lactide initiated by poly(ethylene glycol) in the presence of CaH2. The length of PEO blocks was varied by using parent PEG of different number average degrees of polymerization PEG = 14, 26, and 49, respectively, according to SEC and NMR. SEC and 1H and 13C NMR showed the resulting triblock copolymers did not contain any detectable PLA homopolymer as side product. The use of DMSO-d6 as solvent for NMR analyses, instead of CDCl3, greatly enhanced the resolution and permitted the distinction of the signals due to the last two constitutive units located at both ends of each PLA block. Data obtained by FTIR spectroscopy and X-ray diffractometry suggested that PEO and PLA blocks were phase separated even for copolymers with very short PLA blocks. Optical microscopy and DSC showed that an increase...

Star Polymers and Nanospheres from Cross-Linkable Diblock Copolymers

Abstract: Polystyrene-block-poly(2-cinnamoylethyl methacrylate) (PS-b-PCEMA) formed micelles in THF/cyclohexane or chloroform/cyclohexane mixtures with the PCEMA block as the core. Depending on the relative length of the PS and PCEMA block, star and crewcut micelles were prepared. Upon UV irradiation, the PCEMA block selectively cross-linked to form star polymers and nanospheres from the corresponding star and crewcut micelles. The cross-linked micelles were characterized by solution and solid-state NMR, FTIR, GPC, TEM, and dynamic and static light scattering.

Initiating Systems for Nitroxide-Mediated ``Living'' Free Radical Polymerizations: Synthesis and Evaluation

Abstract: A variety of initiating systems for the preparation of macromolecules by nitroxide-mediated “living” free radical procedures have been prepared and evaluated. The systems can be divided into two classes, unimolecular initiators in which alkylated TEMPO (2,2,6,6-tetramethylpiperidinyloxy) derivatives dissociate to provide both the initiating radical and the stable radical, and bimolecular systems in which a traditional free radical initiator, such as BPO or AIBN, is used in conjunction with TEMPO. For the unimolecular initiators it was found that an α-methyl group is essential for “living” character, while a variety of substituents could be placed on the phenyl ring or the β-carbon atom without affecting the efficiency of the unimolecular initiator. It was also found that the rate of polymerization is approximately the same for both the unimolecular and corresponding bimolecular systems; however, the unimolecular initiators afforded better control over molecular weight and polydispersity.

Evidence of a Novel Side Chain Structure in Regioregular Poly(3-alkylthiophenes)

Abstract: Clarifying the changes in the physical behavior of flexible side chain substituted π-conjugated and other rodlike polymers continues to be an important research topic. For conducting polymers, this side chain addition, when coupled with controlled synthesis, has been shown to dramatically enhance key electronic properties and profoundly alter the macromolecular structures. 1-4 In particular, the ability to specify the extent of poly(3-alkylthiophene) (P3AT) chemical regularity, 4-6 with respect to side chain positioning, has increased the maximum observable conductivities as well as focused attention on the role that side chains play. Regioregular poly(3-dodecylthiophene) (R-P3DDT), with 98% head-to-tail (HT) coupling of the alkyl side chains along the backbone, has conductivities (when doped with an electron acceptor) up to 50 times those seen in the \" regioirregular \" counterpart (IR-P3DDT) having only 75% HT linkages. R-P3DDT, which contains a high proportion of insulating side chains, also exhibits conductivities up to twice those reported for the unsubsti-tuted parent polymer, polythiophene. There are other surprising features as well. P3AT's are also found to be thermo-and solvatochromic. 1,7 Many P3AT samples exhibit semicrystalline lamellar phases exhibiting a thermotropic transition to a liquid crystalline (LC) state. 8,9 While these mesophases may improve proces-sibility, they also have the potential for altering the electronic properties. Consequently, the full range of structure-property interrelationships that side chains impart to these polymers needs to be explored in order to realize their full potential and limitations. 10 The most prevalent crystal structure reported for P3AT's has been studied extensively. 8,9,11-13 These studies support a lamellar base structure in which parallel stacks of polymer main chains are separated by regions filled by the alkyl side chains (see inset of Figure 1). Each layer consists of stacked arrays of the main chains spaced 3.8 Å apart and sequentially displaced, along the skeletal c-axis direction, by one thiophene unit (c/2 or 3.9 Å), resulting in a staggered nesting of the side chains. Structure factor intensity calculations 9 support a model in which these side chains are strongly tilted at an angle such that this side chain nesting approximates a 4.5 Å hexagonal close packing in close correspondence to the average packing of pure alkanes or polyethylene. 14 Overlap (interdigitation) between side chains in neighboring stacks is minimal although there can be ordering of the side chains across the layer-layer interface. 9 Additional phases are described 15-18 but the full structural details are incomplete. In this communication …

Molecular Dynamics Study of Dendrimer Molecules in Solvents of Varying Quality

Abstract: The properties of dendrimers (“starburst” molecules) under varying solvent conditions are studied using molecular dynamics simulations. The dendrimers are found to have a compact (space filling) structure under all solvent conditions, with a radius of gyration which scales with the number of monomers as RG ∝ N1/3. For high generation number dendrimers, there is a distinct region of constant monomer density. The density in this region depends only on the solvent quality and is independent of the generation number. When the contributions of the different generations to the overall density profile are separated, we find that the monomers which belong to the first few generations are stretched and spatially localized. Later generations are less localized and penetrate well into the central regions of the dendrimer. The different primary branches (“dendrons”) of the dendrimers are found to be segregated. The amount of spatial overlap between the different dendrons decreases with increasing generation number of...

Polymer Mobility In Thin Films

Abstract: We present results from a diffusion study of polymer chains in thin films, in which the mobility of chains labeled with a fluorescent dye in a matrix of unlabeled chains has been measured using fluorescence recovery after patterned photobleaching. The results presented here indicate that there is a substantial decrease in the lateral diffusion coefficient for films thinner than ∼1500 A.

Synthesis and Characterization of Model Polyalkane−Poly(ethylene oxide) Block Copolymers

Abstract: We describe the preparation of a new set of amphiphilic block copolymers with well-defined molecular weights and block volume fractions. The synthesis of a variety of polyalkane−poly(ethylene oxide) block copolymers was accomplished by a new polymerization−hydrogenation sequence. Initially, anionic polymerization of either butadiene or isoprene was performed followed by end capping with ethylene oxide. The resulting hydroxyl-terminated polydienes were catalytically hydrogenated to give the corresponding hydroxyl-terminated polyalkanes. These polymeric alcohols were then titrated with potassium naphthalenide to yield the analogous potassium alkoxides. This type of macroinitiator was employed in the polymerization of ethylene oxide. Seventeen polyalkane−poly(ethylene oxide) block copolymers were prepared in near quantitative yields with molecular weights ranging from (1.4 to 8.7) × 103 and poly(ethylene oxide) volume fractions ranging from 0.29 to 0.73. These polymers are model materials for block copolymer...

Efficient Solid-State Photoluminescence in New Poly(2,5-dialkoxy-p-phenyleneethynylene)s

Abstract: A series of novel poly(2,5-dialkoxy-p-phenyleneethynylene)s (PPEs) has been prepared by the palladium-catalyzed cross-coupling polycondensaton of aryl acetylenes and aryl iodides. Different alkoxy side chains including n-hexadecyloxy, n-octyloxy, (2-ethylhexyl)oxy, (2-methylpropyl)oxy, (3-(dimethylamino)propyl)oxy, and (7-carboxyheptyl)oxy groups were attached to the rigid-rod polymer main chain. With this structural concept, polymers having an identical conjugated backbone but different supramolecular structures in the solid state could be achieved. X-ray diffraction measurements on thin films show that the polymers which have sterically hindered side chains are essentially disordered, while those with only linear side chains can form lamellar structures with significant degrees of long-range order. High photoluminescence (PL) quantum yields, up to 0.86 in solution and 0.36 in the solid state, have been measured for the new materials. While the solution quantum yields are independent of the functionaliza...

Determination of Free-Radical Propagation Rate Coefficients of Butyl, 2-Ethylhexyl, and Dodecyl Acrylates by Pulsed-Laser Polymerization

Abstract: Analysis of the molecular weight distributions (MWDs) of polymer produced by pulsed-laser polymerization (PLP) is an effective method for the determination of free-radical propagation rate coefficients (kp). The PLP technique has been successfully applied to many slowly propagating monomers, but kp determination by PLP for faster propagating systems such as the acrylates is more difficult. Experimental conditions for the polymerization must be selected so that laser pulses are the dominant chain-starting and chain-stopping events. Interference from excessive chain-transfer reactions, or from termination reactions which are either too fast or too slow, results in MWDs which are not suitable for kp determination. Guidelines for design of successful experiments are outlined using an extensive n-butyl acrylate data set, from which kp has been determined over the temperature range 5−30 °C; limited data for n-dodecyl acrylate and 2-ethylhexyl acrylate are also presented.

Multiple Morphologies in Aqueous Solutions of Aggregates of Polystyrene-block-poly(ethylene oxide) Diblock Copolymers

Abstract: Aggregates of various morphologies formed from PS-b-PEO diblock copolymers in dilute aqueous solutions were observed by TEM. The morphologies are a function of the soluble block (PEO) length, and, with decreasing PEO content, one obtains progressively, as the dominant morphologies, normal spherical micelle-like aggregates, rods, lamellae and vesicles. The main reason for the morphological transitions is believed to be related to changes of the degree of stretching of the PS blocks in the core regions

A Self-Consistent Mean Field Model of a Starburst Dendrimer: Dense Core vs Dense Shell

Abstract: We examine the equilibrium properties of a starburst dendrimer. A simple Flory theory is developed for the dependence of the radius of gyration of the starburst on its generation. A self-consistent mean field model is solved numerically to verify the Flory theory predictions and elucidate the density profile within the starburst. The density is found to decrease monotonically from the center of the molecule. Structure factors are calculated from the density profiles. Predictions of the model compare favorably with recent experiments and simulations.

Free Volume and the Mechanism of Plasticization in Water-Swollen Poly(vinyl alcohol)

Abstract: Poly(vinyl alcohol), PVOH, films have been studied as a function of water content. The states of water present in the polymer (bound or free) have been characterized and are a function of water content. The effects of water content (and hence state) on the free volume, chain mobility, and glass transition (Tg) behavior have been studied by positron annihilation lifetime spectroscopy, 13C solid state nuclear magnetic resonance, and dynamic mechanical analysis. The addition of approximately 30 wt % water results in a marked increase in free volume cavity size and polymer chain mobility and a corresponding decrease in Tg. The water is in a molecular, nonfreezable state for water additions up to approximately 30 wt %, hence it is postulated that nonfreezing water is responsible for the majority of the plasticization in this system. The plasticization is attributed to the increasing free volume and lubrication provided as the water swells the polymer and disrupts polymer−polymer hydrogen bonding. A limited equ...

Dynamical scaling of DNA diffusion coefficients

Abstract: We present measurements of the Brownian motion of long, fluorescently labeled DNA molecules and observe power-law scaling of the diffusion coefficients with chain length